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The two forms of monoamine oxidase (MAO), A and B, continue to be of major interest to biochemists, pharmacologists, neurologists, and gerontologists. Despite intensive study for more than half a century, unexpected and unique properties of these enzymes continue to come to light. Recent studies have centered on their kinetic mechanism, their unique predilection for substrates related to the neurotoxic tertiary amine MPTP, and their putative role in aging and in the etiology of neurodegenerative diseases. New and potent inhibitors of MAO A and MAO B continue to be developed because of their potential use in clinical medicine. Some are effective in the picomolar range but MAO B from different mammalian species shows remarkable differences in sensitivity to these agents.

Long-chain carnitine acyltransferases are a family of enzymes found in mitochondria, peroxisomes, and endoplasmic reticulum that catalyze the exchange of carnitine for coenzyme A in the fatty acyl-CoA. Conversion of the fatty acyl-CoA to fatty acylcarnitine renders the fatty acid more permeable to the various cellular membranes. The mitochondrial carnitine palmitoyltransferases are considered important in the regulation of mitochondrial beta-oxidation of long-chain fatty acids. However, palmitoylcarnitine produced by peroxisomal carnitine octanoyltransferase or by microsomal carnitine palmitoyltransferase is not different from that produced by the mitochondrial enzyme. Therefore, for there to be control of fatty acid oxidation by the long-chain carnitine acyltransferases, there would have to be some mechanism to coordinately regulate these varied enzymes. The first system of regulation involves inhibition by malonyl-CoA, an intermediate in the synthesis of fatty acids. Malonyl-CoA inhibits long-chain carnitine acyltransferase activity by all three enzymes at similar concentrations in the physiological range. In addition, the mitochondrial and peroxisomal enzymes are known to be regulated at the level of mRNA transcription by a number of shared factors. Although the microsomal enzyme is less well studied, there does, indeed, appear to be a pattern of coordinate regulation for this system.